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Check Point Discovers Media Subtitle Vulnerability Impacting Millions

200 million users could potentially be at risk from vulnerabilities in streaming media players that can enable malicious subtitles to takeover systems.

Security firm Check Point Software Technologies publicly disclosed a new threat vector today in media player subtitles that could have potentially exposed millions of users to security risks. Simply by running a media file that downloads embedded malicious subtitles, Check Point alleges that end-user systems could have been taken over by attackers.

Check Point responsibly disclosed the vulnerability to the impacted media players including VLC, Kodi (XBMC), Popcorn Time and Stremio, and updated players are now available. VLC in particular is a widely used open-source media player that has over 170 million downloads on Windows alone. Media players are also widely used in smart TV platforms and other streaming media devices, with the total number of impacted devices estimated to be 200 million by Check Point.

In this attack the vulnerable media player loads a subtitle from a third party resource to provide a language translation for the user. According to Check Point, subtitles are treated as a trusted source by the media player and are often just text files, which are overlooked by common security tools including antivirus technologies.

Though the attack vector has widespread risk implications, it's not an attack that has been used in the wild to date.

"We have done extensive analysis of millions of public subtitles and as far as we can tell this attack has not been used by anyone yet," Omri Herscovici, Team Leader for Products Research and Development at Check Point told eWEEK.

Herscovici explained that VLC is a program running on user systems that has the potential to do various things once it is installed. He added that enabling subtitles to take control of a system is obviously not supposed to happen, because its not what VLC is programmed to do. The vulnerability that Check Point discovered essentially enables an attacker to act maliciously using VLC's permissions.

The Check Point researchers were also able to determine that it is possible for an attacker to manipulate third party subtitle repositories to enable a malicious subtitle to automatically be downloaded to a media player.

The process by which Check Point figured out that media subtitles could be used to attack user involved multiple tools and steps.

"The vulnerabilities where found through various methods, either by manual auditing of the code, or by creating a fuzzing infrastructure instrumenting the tool AFL (American Fuzzy Lop)," Herscovici said.

Herscovici said that Check Point contacted the VLC developers on April 5. VLC's team was very responsive, he said, and has updated its media player to fix the bugs. For VLC, there were four specific vulnerabilities (CVE-2017-8310, CVE-2017-8311, CVE-2017-8312 and CVE-2017-8313) that have been patched.

For end-users, the best way to limit the risk of any sort of media subtitle attack is to update their media players.

"Since new vulnerabilities come and go all the time, there isn't really bullet proof solution aside from making sure the software is always up-to-date, Herscovici said.